RoHS regulation has come into effect in the Europe, and the use of surface treatment agents containing a hexavalent chromium component such as chromate treatment (surface treatment using hexavalent chromium components) provided for imparting rust inhibitive performance on zinc surfaces has been restricted. Along with the regulation, chromite surface treatment performed for galvanized metal parts has become widespread instead of chromate treatment in industrial fields. However, the chromite treatment has some problems. For example, there are problems that it is difficult to control treatment solutions; the renewal life of treatment solutions is short; and the friction coefficient on the surface of galvanized metal parts subjected to chromite treatment is large. It is necessary to further apply a surface treatment agent for adjusting a friction coefficient in fastener parts such as some galvanized bolts and nuts. In addition, the rust inhibitive performance of galvanized products subjected to chromite surface treatment is inferior to that of galvanized products subjected to conventional chromate treatment, and sometimes relaxation of the rust inhibitive specification required for galvanized parts has also been conducted.
Further, it cannot be prevented that some trivalent chromium is converted into hexavalent chromium by an equilibrium reaction and significant amounts of hexavalent chromium components are detected in the conversion film. For this reason, it is considered that the chromite surface treatment method is a temporary surface treatment method and should be changed to a completely chromium-free surface treatment method in the near future.
Many treatment methods have so far been proposed for completely chromium-free surface treatment. However, in galvanized metal parts provided with a thin coated film obtained by chromium-free surface treatment which is equivalent in thickness to that obtained by chromate treatment, the rust-inhibitive performance has not yet reached the practical use. In the case of applying top coating, the surfaces of galvanized metal parts should be subjected to chromium-free surface treatment with a film having a thickness exceeding 10 μm to achieve the required rust-inhibitive performance. However, there is no chromium-free rust-inhibitive surface treatment only with thin coated films for galvanized metal parts showing rust-inhibitive performance which is not inferior to that obtained by chromate treatment, except for the chromium-free rust-inhibitive surface treatment which the present inventors have previously proposed.
In Patent Document 1, the present inventors have proposed a chromium-free rust-inhibitive surface treatment agent which is applied onto the surfaces of galvanized metal parts to form a thin siliceous film which can suppress the generation of red rust for a long period of time. Nano-sized powders of titanium oxide which are subjected to dispersion treatment and have an averaged primary particle size of 70 nm or less have been blended in an effective amount in the chromium-free rust-inhibitive surface treatment agent.
Further, in Patent Document 2, the present inventors have proposed a chromium-free rust-inhibitive surface treatment agent for zinc surfaces composed mainly of an alcoholic solution of alkoxysilane oligomer having a specific weight-averaged molecular weight. When the chromium-free rust-inhibitive surface treatment agent is applied onto the zinc surfaces of galvanized products and the like to form a thin siliceous film, the generation of white rust can be suppressed for a long period of time.
In utilizing the chromium-free rust-inhibitive surface treatment agent composed mainly of an alcoholic solution of alkoxysilane oligomer, when galvanized metal parts are subjected to an activation treatment in which the metal parts are immersed in a diluted aqueous nitric acid solution (a pickling process performed as pre-treatment of chromate treatment) and then subjected to chromium-free surface treatment, the rust-inhibitive performance is poor in many cases. For this reason, galvanized metal parts not subjected to nitric acid activation treatment (pickling) are water-rinsed and dried, and then they are subjected to chromium-free rust-inhibitive surface treatment.
Further, when the chromium-free rust-inhibitive surface treatment agent composed mainly of an alcoholic solution of alkoxysilane oligomer is applied onto as-galvanized bolts (without chromate treatment) obtained from several galvanizers, there is a problem that the rust-inhibitive performance in the generation of white rust is greatly changed depending on which galvanized bolt is applied.
As a method which can prevent this problem, in Patent Document 3, the present inventors have proposed rust-inhibitive treatment in which metal parts are previously subjected to chromium-free conversion coating and then applied with a chromium-free rust-inhibitive surface treatment agent composed mainly of an alcoholic solution of alkoxysilane oligomer. However, the surface treatment needs to add at least one treatment process and therefore cannot satisfy the request of users who wish to perform surface treatment in a simple process.
Thereafter, when galvanized metal parts such as bolts are applied with this chromium-free rust-inhibitive surface treatment agent and the coated metal parts are stored for about 1 year, there arises a problem that: crazing occurs in the coated film; a phenomenon that crazing occurs in the coated film and subsequently the film is peeled off in some portion where a little thick coated film (3 μm or more) is applied; and white powders are observed on the galvanized surface and it seems as if white rusting is generated.
Patent Document 4 has not mentioned rust-inhibitive performance, but has disclosed a coating composition in which an alkoxysilane is hydrolyzed by adding an acid catalyst and water and condensation-polymerized while evaporating alcohol and water. The every alkoxysilane used as a raw material of the coating composition in Examples is an alkylalkoxysilane. In addition, it is described that a chelate compound of zirconium, titanium or aluminum is blended into this composition. However, there is an example in which a zirconium chelate compound is blended (see Example 7 in the same Patent Document 4), but there is no example in which an organic chelate titanium compound is blended or there is no example in which metal parts with zinc surfaces are coated.
Further, Patent Document 5 has disclosed a silica-based protection coating solution in which an alkoxysilane and a titanium alkoxide are hydrolyzed in an alcoholic solution using acetic acid as a catalyst and then condensation-polymerized. In the examples, examples of an object to be protection-coated include CFRP (carbon fiber reinforced plastics) in addition to metal parts such as titanium. In addition, as the alkoxysilane used as a raw material of a protection coating solution, an alkoxysilane having an epoxy functional group and an alkoxysilane having an amino group (exhibiting basicity) are used. Since an amino group functions as a basic catalyst for proceeding with a cross-linking reaction between alkoxysilane oligomer molecules in the solution, there is a disadvantage that the solution is apt to gelated. In Example 4 in the same Patent Document 5, after the protection coating solution is applied onto a hot-dip galvanized steel plate using a mixed coating solution of alcohol and water, the adhesion of the coated film to the substrate is evaluated, but the rust-inhibitive performance is not examined.    Patent Document 1: JP 2005-097719 A    Patent Document 2: JP 2005-264170 A    Patent Document 3: JP 2006-225761 A    Patent Document 4: JP 07-157715 A    Patent Document 5: JP 2003-160759 A